The present invention relates to a coating composition for preparing a back coating film of a mirror.
To protect a mirror having a silver mirror film, the silver mirror film is coated with a metallic protective film and then the metallic protective film is coated with the back coating film. In fact, the back coating film is provided, for example, for preventing breakage of the mirror and increasing mechanical strength and corrosion resistance of the mirror against water, water vapor, various gases, acids, alkalis, detergents and the like. A conventional coating composition for preparing the back coating film contains, as a binder, lacquer, alkyd resin, melamine alkyd resin, phenol alkyd resin or the like, and, as a corrosion-inhibitive pigment, red lead, lead cyanamide, basic lead carbonate, basic lead sulfate, calcium plumbate or the like. However, this conventional composition is not able to provide a back coating film which is proof against a severe environment.
In view of this, for example, JP-A-63-184701 and JP-A-63-184702 disclose coating compositions for preparing a back coating film of a mirror so as to improve corrosion resistance, durability and workability of the mirror. The coating composition of the former publication contains a binder including an epoxy resin, a hardener, a rosin and/or a petroleum resin. The coating composition of the latter publication contains a binder including an epoxy resin, a hardener and a coumarone indene resin. The rosin contains abietic acid, L-pimaric acid and the like, and thus is considerably high in acid and iodine values. The petroleum resin is a copolymer of hydrocarbon compounds having unsaturated bonds of a petroleum fraction. The petroleum resin takes various forms such as aromatic type, aliphatic type, aliphatic-aromatic and alicylic type, and has active double bonds as is apparent by bromine and iodine values and the like. The coumarone indene resin is a copolymer of coumarone, indene, styrene and the like of naphtha fraction and has active double bonds. Thus, the coating compositions of the above publications contain resins having active double bonds. With this, the back coating film tends to be undesirably oxidized with the passage of time. Furthermore, it is known that a conventional two-package coating composition containing a hardener such as the compositions of the above publications is superior in adhesion and chemical resistance, but is unsatisfactory in workability to cut the mirror. That is, the back coating film tends to peel off when the mirror is cut.
JP-A-3-75006 and JP-A-3-75007 disclose coating compositions for preparing a back coating film of a mirror, which contain an amino modified epoxy resin prepared by reacting an epoxy resin with amino-alcohols, so as to improve corrosion resistance, durability and workability of the mirror. However, the back coating films prepared from these coating compositions are not substantially improved in chemical resistance and the like due to the lack of hardener in the coating composition.